[incremental] Isolate the "used trait imports" part of typeck tables into a distinct query

[nikomatsakis]

As part of #45208, one place where typeck_tables_of is used that cannot easily be removed is this call from check_unused. What this code is doing is aggregating the set of trait imports that are used by all the function bodies in the crate. Due to glob imports and the like, it is hard to isolate which function body may make use of which trait import. Also, this information is only available from type-checking. So this will require serializing some amount of incremental data in between sessions. But since we don't want to save all of the typeck data, we can at least minimize the amount of data.

The idea is to introduce a new intermediate query used_trait_imports(D) for some def-id D. It will have the return type Rc<DefIdSet> and will return the used_trait_imports field from typeck_tables_of(D).

The steps are as follows:

• Change the type of used_trait_imports to be Rc<DefIdSet> so it can be cheaply cloned. Make the code compiled.
• Introduce the used_trait_imports query as defined above; it should return tcx.typeck_tables_of(key).used_trait_imports.clone().
• Modify these lines to use your new query instead of typeck_tables_of.

[cjkenn]

I think I'd like to take this on! Would be my first contribution to rust but you've laid the steps out pretty clearly and provided a lot of context so I think I would be able to work on it

[wesleywiser]

I think this issue can be closed now.